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TPS2111A: power mux continuously switches beetween two inputs even if one is not connected

Andrea Milluzzo
Prodigy 20 points
Part Number: TPS2111A
Other Parts Discussed in Thread: TPS2111,

Hi all,
I'm testing a new design and I'm facing the following problem: I set up the TPS2111 in order to have priority on IN1 when both input sources are plugged in (IN1 = 5V from SEPIC power supply, IN2 = 5V from USB, D0 connected to ground and D1 connected to IN1).
When IN1 is connected it's all ok, but when I connect ONLY IN2 (SEPIC input unplugged), after a while, the power mux gets crazy and starts to switch beetween IN1 e IN2 continuously. To prove that, I can see at the input of IN1 1.8/1.9V even if nothing is connected, which would trigger the mux to switch input. I wonder if it is possible that TPS could apply that 1.8/1.9V from the IN1 pin. 
Attached you can se the schematic:

Anyone has faced the same problem or could help me, please?
Thank you in advance.

  • 0
    TI__Genius 13460 points

    Hi Andrea,

    Welcome to E2E!

    I had never seen this problem before so it does look a bit odd.

    Have you only seen this behavior in one device or have you tested multiple devices and seen the same behavior?

    Is there anything that triggers the switchover or does it simply occur over time?

    If it only occurs over time, how much time does it take for the error to occur?

    Does it always take the same amount of time?

    Best regards,

    Andy Robles

  • 0 in reply to Andy Robles
    Prodigy 20 points

    Hi Andy, Thank you for answering!

    I substituted the component on the same board but the problem was still persisting. I then tried by removing 100nF capacitor on IN1 and the TPS managed to work, I don't know exactly why, since the 100nF capacitor is recommended on the datasheet and obviously it cannot raise that node; maybe something related to the internal stability of the chip and the filters at the inputs?
    If I re-solder the capacitor the problem re-appear.

    btw, regarding your questions:

    Andy Robles said:

    Is there anything that triggers the switchover or does it simply occur over time?

    If it only occurs over time, how much time does it take for the error to occur?

    Does it always take the same amount of time?

    the switchover started simply over time apparently: the board was powered through USB cable and after random time (sometimes 30 seconds, sometimes right after powering...) I can notice the LED D7 connected to IN1 lighting up whitout no input connected and noise (like the acustic noise of an inductor) coming form the TPS.

    Thank you again for your answer,
    Best regards,

    Andrea Milluzzo

  • 0 in reply to Andrea Milluzzo
    TI__Genius 13460 points

    Hi Andrea,

    Internally the D1 pin will always have a 1uA pull coming from either IN1 or IN2. This pull up-along with the capacitor could be causing the erratic behavior observed in your circuit when connecting D1 directly to IN1, but it is odd behavior since you do have the R14 1kOhms pull down resistor.

    Have you tried using configuration 15 in the datasheet of the TPS2111A since this is the configuration we recommend for priority selecting of a source?

    Using the configuration in Figure 15 would allow to not have to worry about the D1 and D0 pins and will allow you to set a threshold for the switchover in your circuit using the VSNS pin.

    Best regards,

    Andy Robles